[igraph] Why are unconnected subgraphs not considered by motif counting functions?

[Szabolcs Horvát]

Dear All,

igraph's motif counting functions (motifs()) will not count
unconnected subgraphs and always return NA for the 1st, 2nd and 4th
element of the result (for 3-motifs).

The reasoning is that (quoting from the docs):

>  Note that for unconnected subgraphs, which are not considered to be motifs, 
> the result will be NA.

Is there any _technical_ reason for this behaviour?  I understand that
according to some definitions, unconnected subgraphs are not
considered motifs, but in many applications it is still useful to
count _all_ size-k subgraphs.

It would be useful if motifs() returned all counts.  It is much easier
to simply ignore those I don't want (when I don't want them) than
having to separately count the missing ones.

Szabolcs